In marine propulsion drives, an outer radial sealing ring is normally applied on the propeller shaft for preventing sea water from entering the drive housing. An inner radial sealing ring is also applied for preventing transmission lubricants from leaking out into the water. The radial sealing rings traditionally comprise lips which respond to external water pressure by pressing harder against the propeller shaft. In this way excessive external pressure on the sealing ring results in largely increased frictional wear of the sealing ring, which in turn may lead to undesired leakage of sea water into the drive housing.
In drives with pulling propellers, the submerged drive housing is often broader than the propeller hub. Consequently, a front end shoulder portion on the drive housing is formed at the transition between the hub and the housing. As water flows downstream along the periphery of the propeller hub, a significant dynamic pressure build-up is created locally as water is forced to deflect radially outwards past the shoulder portion of the drive housing, especially at high speed.
A problem with known pulling propeller drive designs is that this sharply increased pressure at the transition is also felt by the pressure sensitive outer radial sealing ring, leading to rapid wear of the ring and eventually, leakage.